Today when I got home and awoke my MacBook Air (running OS X 10.11.6) from sleep by lifting up the lid to login into it, I noticed the little Screen Sharing icon in the top-right corner (to the left of the battery percentage). I took this screenshot the moment I noticed it, but as soon as I actually logged in the icon disappeared. I am worried because I have always had Screen Sharing disabled in System Preferences and it has never been enabled, even if I try to start a Screen Sharing session from another computer to mine (I tested it through Messages on another mac) it is outright rejected. I also could not find any logs to indicate that there was a Screen Sharing session active or attempted. Is there any way to check if my system has been accessed, and if so by whom?